1. Field of the Invention
This invention relates generally to the fabrication of dynamic random access memories (DRAM""s), particularly such memories in which the memory storage device is a deep trench capacitor and wherein individual memory cells are electrically isolated from each other by isolation trenches.
2. Description of the Related Art
Typical dynamic random access memories (DRAM""s) consist of densely packed arrays of memory cells which are themselves comprised of a charge storage device coupled to a charge accessing device. The charge storage device is usually a capacitor and the accessing device is usually a single MOS field effect transistor (MOSFET). The transistor can be connected at its source to one plate of the storage capacitor, at its drain to a conducting bit line and at its gate electrode to a conducting word line. In a normal operation in which a 1 or a 0 is written on or read from a particular memory cell, the particular cell is selected by choosing the appropriate pair of intersecting word and bit lines. Applying a given potential to the word line turns on the access transistor and a given charge applied to the bit line will then be deposited on the capacitor plate and stored. Conversely, during reading, the word line again activates the transistor and this time the presence of the charge in the capacitor is sensed by appropriate circuitry and identified as a 1 or 0.
A major problem associated with the design of increasingly dense arrays of memory cells is the necessity of decreasing the surface area of the storage capacitors while not decreasing their capacitance. Chatterjee et al. (U.S. Pat. No. 5,208,657) describes the difficulties of using planar capacitors as storage units in a DRAM module having a 5 volt power supply. Such a capacitor would require a capacitance of approximately 50 femtofarads (fF) and, in consequence, a capacitor area of approximately 20 square microns. Not only is this area overly large for the most densely packed circuits, but it presents an unacceptably vulnerable target for alpha particles which are a causal mechanism for soft error rates (SER). Chatterjee discloses the use of a trench capacitor, which is a capacitor formed in a trench or cavity that extends vertically into the substrate of the integrated circuit that can be produced by a variety of etching mechanisms. Such capacitors gain plate area and, hence, capacitance, through an increased vertical extension rather than by horizontal extension. One plate of such a capacitor is defined by the surface of the inner wall of the doped region of the substrate within which the trench is formed. Although this inner wall forms the plate boundary, charge can in fact also be stored within a depletion region formed beneath the wall surface and extending into the doped substrate. The other plate of the capacitor, which can also be a storage plate, is a conductive core that is deposited within the trench. An oxide layer is first formed over the inner trench wall to serve as a dielectric medium and to insulate one plate from the other. The prevalence of such trench capacitors within the practice of the prior art is illustrated by the work of Kato et al. (U.S. Pat. No. 4,907,047), which teaches a method of forming a memory cell comprising a storage capacitor having a first electrode formed in the side and bottom wall portions of a primary cavity and having a second electrode formed over a dielectric film which covers said first electrode. Motonami (U.S. Pat. No. 5,185,284) describes a xe2x80x9cgroovexe2x80x9d type capacitor (which is essentially a trench capacitor) which is circumscribed by a second capacitor to increase the overall capacitance of the combination. Cunningham (U.S. Pat. No. 6,177,697) describes a trench capacitor partially surrounded by a shallow trench isolation region (STI). The substrate is then doped through the inner surface of the trench capacitor, which is then lined with an oxide and filled with a polysilicon core to form a capacitor plate. A second capacitor plate is formed by the doped surface of the inner wall of the capacitor trench. Alsmeier et al. (U.S. Pat. No. 5,793,075) describes a deep trench capacitor within a lightly doped substrate wherein an inversion layer beneath the trench serves as one plate. Sakamoto et al. (U.S. Pat. No. 5,574,621) discloses a trench capacitor in which a plurality of conductor filled trenches connected by a conducting layer comprise a single bottom electrode and a second conducting layer, disposed over a dielectric layer, comprises a top electrode. Hoenigschmid et al. (U.S. Pat. No. 6,037,620) discloses a DRAM cell comprising a shallow isolation trench (STI), a storage trench capacitor partially overlaid by the STI and an access IGFET transistor connected to the perimeter of the capacitor by a conduction path that utilizes a buried strap interconnection.
The need to increase the density of memory cells and to integrate them with logic devices leads to complex fabrication processes. In particular, the fabrication of large DRAM arrays is complicated by a plurality of dielectric layer depositions to form gate and capacitor dielectrics and polysilicon depositions which are necessary to form the word and bit line connections to each cell and to provide connections between the access transistors and their associated storage capacitors. Even with the use of trench capacitors to reduce surface areas, the spacing between capacitors is limited by charge diffusion considerations. It is the purpose of the present invention to provide a method of isolating densely packed memory cells from each other while doing so in an efficient and cost effective manner that focuses on the minimization of dielectric and posysilicon depositions. In particular, the novel approach of forming the trench capacitor and the access transistor within a double well in which an isolation trench has already been formed provides an efficient method for forming cell plate connections having advantageous resistance properties.
A first object of this invention is to provide a method for forming one or a plurality of densely packed dynamic random access memory (DRAM) cells that can be efficiently and cost-effectively integrated with associated logic circuitry.
A second object of this invention is to provide such single or integrated DRAM cells that are characterized by low noise and soft error rates (SER).
A third object of this invention is to provide a method for forming one or a plurality of DRAM cells with high storage capacitance and low cell area.
A fourth object of this invention is to provide a method for forming DRAM cells and circuitry which can be diminished in dimension to levels exceeding that required by the 0.18 micron generation of devices and beyond.
A fifth object of this invention is to provide a method for forming such DRAM cells that produces a low cell plate connection resistance.
A sixth object of this invention is to provide a method for forming a DRAM memory cell that can be independently biased.
In accord with the objects of this invention there is provided a method for forming a single DRAM memory cell or an array of such DRAM memory cells in a semiconductor substrate. Each such cell includes a trench capacitor as a charge storage mechanism having a high capacitance and small surface area, a MOSFET as the accessing mechanism for storing and retrieving charge from the storage capacitor and a shallow isolation trench (STI) for electrically isolating one cell from another. Further in accord with the objects of this invention there is provided a method for forming the isolation trench, the capacitor and the transistor within a double well structure formed in the substrate, said well structure comprising a relatively shallow doped well of a first dopant type (the upper well) formed within a deeply implanted doped well of a second dopant type (the lower well). The isolation trench is contained within the upper well. The capacitor trench extends through the upper well and terminates within the lower well, wherein the trench bottom and the portion of its inner sidewall surface within said lower well forms one capacitor plate (a top plate) and the well itself provides a low resistance connection to said plate. A preferred embodiment of this invention discloses a double well which is a relatively shallow P-well substantially contained within a deep N-well formed in a silicon substrate. Further in accord with the objects of this invention, when more than one storage capacitor is to be formed within the same well structure in forming an array of memory cells, there is provided a method for separating the trench capacitors of neighboring memory cells by the formation of shallow trench isolation (STI) regions within the well structure. Yet further in accord with the objects of this invention, there is provided a method of forming the trench capacitor so that a substantial portion of the sidewalls and the bottom of the capacitor trench lie within the lower well, said portions thereby forming the top plate of the capacitor, while the bottom plate of said capacitor is formed by the deposition of a conducting layer which completely fills the interior of the trench, but is separated from the interior surface of the trench sidewall by a dielectric layer which contacts all regions of the trench sidewalls and base except a region within the upper well from which the dielectric layer has been removed and which the bottom plate thereby contacts electrically. Within the preferred embodiment, the conducting layer is a layer of doped polysilicon. Yet further in accord with the objects of this invention there is provided a method of forming an access transistor of the MOSFET type immediately adjacent to the trench capacitor, the gate electrode of said transistor being formed on a gate dielectric layer over the substrate surface above the double well and the source and drain of said transistor being formed beneath the gate electrode within the upper well of the double well formation and said transistor advantageously making direct electrical contact with the bottom plate of the trench capacitor by means of an doped region, autodiffused from the doped polysilicon bottom plate, that contacts said bottom plate through an upper region of the interior trench wall surface from which the dielectric layer has been removed. Still further in accord with the objects of this invention and to facilitate fabrication of said memory cells in an efficient and cost effective manner, there is provided a method of forming both the bottom capacitor plate, which is the conducting layer filling the trench, and the gate electrode and word line of its access transistor, using a single deposition of a conducting material, which is doped polysilicon in the preferred embodiment. Further in accord with the objects of the present invention there is provided a method of forming said DRAM structures while employing a single oxide growth process to form both the transistor gate oxide and the capacitor dielectric layer. Still further in accord with the objects of this invention there is provided a method of forming said DRAM structures and associated circuitry while using a minimum number of photolithography masking steps.